Abstract
Clear physiological differences exist in tolerance to the special conditions within the gastro-vascular cavity of sea anemones between species of lysianassid amphipods habitually found living in this habitat and closely related free-living species. The free-living amphipods, Orchomenopsis obtusa and Tryphosa horingii, are quickly killed by toxic substances produced by the sea anemone, both in in vivo experiments and in in vitro experiments using extracts from the mesenterial filaments, while the species associated with sea-anemones, Aristias neglectus and Onisimus normani, survive for long periods. The toxic substances are large organic molecules, probably of a protein nature. The different tolerances to the toxic substances seem to be genetically determined and not acquired during the life of the individual, in contrast to the situation in the anemone-fishes.